1. Field of the Invention
The present invention relates to aircraft alert and collision avoidance systems and more particularly to a method and apparatus to permit the pilot of an aircraft to proceed when one of the antennas of the system fails.
2. Description of the Prior Art
Aircraft traffic alert and collision avoidance systems are well known in the art. A particularly useful system is sold by Honeywell Inc. identified as TCAS II. The TCAS II system employs two transponders and two antennas (an upper antenna and a lower antenna). Either one of the transponders may be switched to the antennas to provide a dual transponder system. The TCAS II system has several modes of operation which are pertinent to the present invention. A first is when a switch on a TCAS II/transponder control panel is turned to a "STBY" (standby) position in which both transponder systems are not activated. A second is when the switch is turned to "XPDR ON" (transponder on) in which case the system has the selected transponder active. A o third is when the switch is in a "TA" (Traffic Alert) position in which case the pilot is alerted whenever any aircraft having Mode-S or Mode-C transponders are about to enter into a first predetermined envelope of airspace (the "caution area"). A fourth is when the switch is in a "TA/RA" (Traffic Alert/Resolution Advisory) position in which case the pilot is alerted whenever aircraft having Mode-S or Mode-C transponders are about to enter into a second predetermined envelope of airspace (the "warning area"). TCAS II can also detect aircraft having Mode-A transponders, but will display them without altitude indication.
Both the upper and lower antennas of the transponder must be operable per FAA requirements to permit the "warning area" display of the KA information to the pilot but only one antenna (either upper or lower) is required for the display of the "caution area" or TA information to the pilot.
Under FAA dispatch requirements and worldwide aircraft rules, aircraft are not permitted to take off without an operating transponder system (a transponder and antenna combination In the prior TCAS II system, if one transponder system failed, the pilot was informed and would switch to the other transponder. If the problem in the first transponder system had been internal, then the second transponder would be good and the failure indication would go away so that the pilot could take off. However, if the problem in the first transponder system had been an antenna failure, the failure warning would not go away when the pilot switched to the second transponder because TCAS II was set up to provide the failure indication if either antenna failed even though the other antenna was good even when in the TA only mode of system operation. Accordingly, the pilot had no alternative but to remain on the ground until the problem was found and repaired.